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Atomic Force Microscopy Study of the Interaction of DNA and Nanostructured β-Gallia Rutile Nathan Empie* and Doreen Edwards School of Engineering, New York State College of Ceramics, Alfred UniVersity, Alfred, New York 14802 ReceiVed January 20, 2006. In Final Form: June 21, 2006 The ability to attach DNA molecules to solid planar substrates is desired for imaging the molecule and for building DNA-mediated nanostructures. The deposition of DNA on [001] rutile and β-gallia rutile (BGR) substrates from buffer solutions containing various divalent cations was studied using tapping mode atomic force microscopy (AFM). β-Gallia rutile intergrowths were prepared by spin-coating gallium isopropoxide onto [001]-oriented TiO2 single-crystal slabs and heating above 1350 °C for >24 h, resulting in the formation of intergrowth lines along the {210} planes in the parent rutile structure. Rutile and BGR intergrowth substrates were exposed to various buffered solutions containing DNA and the following divalent cations: Ca(II), Co(II), Cu(II), Fe(II), Mg(II), Mn(II), Ni(II), and Zn(II). Among all the cations examined, only Ni(II) resulted in the attachment of DNA on the rutile surfaces. DNA attachment to BGR surfaces was strong enough to allow AFM imaging when the deposition buffer contained one of the following cations: Co(II), Mg(II), Mn(II), Ni(II), and Zn(II). For all of these cations, DNA attachment occurred preferentially, but not exclusively, along BGR intergrowth lines. When buffers without cation additions and those containing Ca(II), Cu(II), and Fe(II) were used, DNA failed to bind the BGR surfaces strongly enough to allow AFM imaging. The mechanism(s) by which DNA attaches to the BGR surface is (are) not well understood but may involve the incorporation of divalent cations at the tunnel sites of the BGR intergrowths.
Introduction The ability to attach DNA molecules in patterned arrangements on solid planar substrates is desired for building DNA-mediated nanostructures for optical and electronic devices. A variety of techniques, most involving the covalent bond between sulfur and gold, have been reported for attaching DNA to a planar substrate. Typically, these techniques require the functionalization of DNA with thiol or disulfide groups and sophisticated manipulation methods for fabricating patterns.1-5 The attachment of DNA to planar substrates is also required for atomic force microscopy (AFM) studies of the biomolecule. Mica has emerged as the preferred substrate for such studies because of its flatness. The attachment of DNA to mica surfaces is significantly aided when the deposition buffer contains divalent or multivalent cations, where varying cation species and cation concentration has been shown to alter the binding of DNA to the mica surface.6-14 Thundat et al. observed the attachment of DNA to mica surfaces with AFM in air when the mica was chemically modified with Co(II), La(III), Mg(II), and Zr(IV) and noted that superior images were recorded with the divalent species.13 Hansma et al. investigated AFM imaging of DNA bound on mica in an aqueous buffer, where DNA was firmly attached in the presence of Ni(II), Co(II), and Zn(II).14 The mechanism by which DNA is attached to mica surfaces in the presence of divalent cations is not well understood; however, it has been suggested that crystallographic sites, often referred to as cavities, on the mica surface may play a role. It has been speculated that divalent (1) Demers, L. M.; et al. Science 2002, 296, 1836. (2) Schwartz, P. V. Langmuir 2001, 17, 5971. (3) Yourdshahyan, Y.; Zhang, H. K.; Rappe, A. M. Phys. ReV. B 2001, 63, 4. (4) Gueroui, Z. Colloids Surf., B: Biointerfaces 2004, 33, 53. (5) Medalia, O.; et al. Ultramicroscopy 2002, 90, 103. (6) Pietrement, O.; et al. Langmuir 2003, 19, 2536. (7) Vesenka, J.; et al. Ultramicroscopy 1992, 42-44, 1243. (8) Pastre, D.; et al. Biophys J. 2003, 85, 2507. (9) Hansma, H. G.; et al. Nucleic Acids Res. 1996, 24, 713. (10) Sanchez-Sevilla, A.; et al. Ultramicroscopy 2002, 92, 151. (11) Rippe, K.; Mucke, N.; Langowski, J. Bioforum Int. 1997, 1, 42. (12) Bezanilla, M.; et al. Langmuir 1995, 11, 655. (13) Thundat, T.; et al. Scanning Microsc. 1992, 6, 911. (14) Hansma, H. G.; Laney, D. E. Biophys. J. 1996, 70, 1933.
cations sitting in the surface cavities act as an electrostatic bridge between the negatively charged mica surface and phosphate backbone.14-17 The amount of DNA adsorbed on mica can be roughly controlled, but without additional surface functionalization, the molecules attach randomly to the surface.15,18 In this work, we report on the attachment of DNA to β-gallia rutile (BGR) substrates. We have selected BGR substrates because they possess features that, with further development, may enable the self-assembly of DNA in patterned arrangements without the need for sophisticated manipulation techniques. BGR substrates possess surface sites which may facilitate the binding of DNA and which align to form patterns related to the material’s crystal structure. β-Gallia rutile (BGR) intergrowths form when gallia (Ga2O3) subunits are inserted into [001]-oriented rutile (TiO2) along {210}r planes. The lattice mismatch between the two crystal structures leads to the formation of one-dimensional tunnels aligned along {210}r and running perpendicular to the rutile surface. These tunnels are approximately 2.5 Å in diameter, making them a suitable host for small to medium-sized cations.19 The separation of β-gallia regions can be controlled by the ratio of gallium to titanium governed by the formula Ga4Tin-4O2n-2.19-27 As an example, Figure 1 shows the n ) 25 (Ga4Ti21O48) member of the intergrowth series. By increasing or decreasing the n value, the distance separating the rows of tunnel-rich β-gallia regions increases or decreases, respectively.25 At low n values, the BGR intergrowths are considered compounds that are distinctly different phases from rutile and β-gallia. At higher n values, the intergrowth structures are often viewed as linear defects in the rutile structure. Incorporation of cations in the tunnel sites has (15) van Noort, S. J. T.; et al. Biophys. J. 1998, 74, 2840. (16) Stigter, D. Biopolymers 1998, 46, 503. (17) Rivetti, C.; Guthold, M.; Bustamante, C. J. Mol. Biol. 1996, 264, 919. (18) Bustamante, C.; et al. J. Biol. Chem. 1999, 274, 16665. (19) Empie, N. H.; Edwards, D. D. Solid State Ionics 2006, 177, 77. (20) Bursill, L. A. Acta Crystallogr. 1979, A35, 449. (21) Bursill, L. A.; Stone, G. G. J. Solid State Chem. 1981, 38, 149. (22) Gibb, R. M.; Anderson, J. S. J. Solid State Chem. 1972, 5, 212. (23) Kahn, A.; et al. J. Solid State Chem. 1986, 65, 377. (24) Kamiya, S.; Tilley, R. J. D. J. Solid State Chem. 1977, 22, 205. (25) Lloyd, D. J.; Grey, I. E.; Bursill, L. A. Acta Crystallogr. 1976, B32, 1756. (26) Stone, G. G.; Bursill, L. A. Philos. Mag. 1977, 35, 1397. (27) Edwards, D. D.; et al. J. Solid State Chem. 2000, 150, 294.
10.1021/la060206z CCC: $33.50 © 2006 American Chemical Society Published on Web 08/01/2006
AFM Study of DNA-BGR Interactions
Langmuir, Vol. 22, No. 18, 2006 7659 of buffer was reduced to 10 µL and applied to both salt-pretreated and untreated BGR surfaces. Following AFM imaging, the dried 10 µL samples were rinsed with distilled water and reimaged. Atomic force microscopy was performed using a Nanoscope IIIa instrument (Digital Instruments, Santa Barbara, CA). All imaging was collected in tapping mode using silicon cantilevers (122 µm long, Vecco Nanoprobe, Santa Barbara, CA) and a scan rate of 1.25 Hz. For each sample, an origin was selected and the surface was mapped with Cartesian coordinates. Mapping the sample surface allowed reimaging of the same area following different surface treatments.
Results and Discussion
Figure 1. Projection of the β-gallia rutile (Ga4Ti21O48) structure along [001] of the parent rutile structure, after ref 24. Twodimensional projections of TiO6 octahedra are shown in gray. Projections of GaO6 octahedra and GaO4 tetrahedra are shown in black.
been observed. For example, the structure of the one-dimensional ion conductor Na0.7Ga4.7O0.3O8 can be viewed as a derivative of the n ) 5 BGR structure (Ga4TiO8) by replacing a portion of the Ti4+ cations with Ga3+ and inserting an equal number of Na+ cations in the tunnel sites.19,28 On the basis of cation/DNA interactions observed on mica surfaces, we have speculated that incorporation of divalent cation species at the tunnel sites may provide preferential locations for DNA attachment. The aim of this work was to examine the validity of that speculation. Herein, the observed interaction of DNA with BGR surfaces in the presence of divalent cations via AFM is reported. Experimental Procedure Single-crystal TiO2[001]-oriented substrates (5 × 5 × 0.5 mm) were obtained from MTI Corp. (Richmond, CA). To prepare the BGR substrates, a gallium isopropoxide solution (0.15 M) was prepared as described by Li et al.29 and applied to the surfaces of the rutile substrates via spin-coating for 25 s at 2500 rpm. The coated substrates were dried for 24 h prior to heating. Initial experiments were performed on two samples to determine the appropriate heating cycles to form suitable BGR substrates. Additional samples were heated at 900 °C for 3 h and then at 1350 °C for 96-480 h. A 1 µg/µL, 100 base pair (bp) DNA ladder (Invitrogen, Carlsbad, CA) containing double-stranded DNA molecules of 16 sizes (ranging from 100 to 1500 bp and 2072 bp) was diluted to 0.5 ng/µL in buffers of 10 mM HEPES (4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid), pH 7, and 1 mM concentrations of various salts (CaCl2, CoCl2, CuCl2, FeCl2, MgCl2, MnCl2, NiCl2, and ZnCl2). A blank sample, containing no salt additions, was also prepared. BGR and rutile substrates were exposed to the DNA buffer solutions using several different application methods and imaged using atomic force microscopy. Initially, samples were prepared by exposing the substrates to 20 µL of DNA buffer solutions for 10 min. The sample surfaces were dried via wicking with filter paper and compressed air. A second series of samples was first pretreated with a 10 mM divalent salt solution for 5 min, rinsed with distilled water, and then exposed to 20 µL of DNA buffer solutions for 10 min before wicking and drying. For the third series of samples, 20 µL of DNA buffer solution was applied to BGR surfaces and allowed to dry completely prior to imaging. Due to complications, the volume (28) Chandrashekar, G. V.; Bednowitz, A.; La Placa, S. J. In Fast Ion Transport in Solids; Vashishta, P., Mundy, J. N., Shenoy, G. K., Eds.; North-Holland: Amsterdam, 1979; p 447. (29) Li, Y.; et al. Sens. Actuators 2003, B93, 431.
β-Gallia Rutile Synthesis. To determine the heating conditions required to fabricate suitable BGR substrates, trials were conducted on two samples. The first BGR test sample (sample A) was imaged after sequentially heating it at increasing temperatures ranging from 900 to 1350 °C for 24-96 h, as shown in Figure 2. Intergrowth formation, as evidenced by the formation of lines along the {210} planes, did not occur until the sample was heated at 1350 °C. The chemistry of the BGR sample surface was confirmed using Auger electron spectroscopy. As shown in Figure 3, gallium was observed primarily along the {210} intergrowth lines, which can also be observed by scanning electron microscopy. In general, the intergrowth lines were randomly oriented along {210} planes, but in some regions, a relatively uniform parallel alignment of the intergrowth lines was observed (Figure 4). Parallel alignment was predominantly observed in areas adjoining surface cracks. The nature of the surface cracks is not well understood, but it is speculated that elevated levels of gallium oxide deposited in the cracks may act as a continuous source of gallium oxide that promotes parallel orientation of the intergrowth lines. The micrographs in Figures 3 and 4 demonstrate the potential for achieving patterned arrangements of intergrowth lines on the BGR surface. A second test sample (sample B) was sequentially heated at 1350 °C and imaged by AFM. As illustrated in Figure 5, the relative density of observed intergrowth lines initially increased and then decreased with continued heating. The so-called relative density of intergrowth lines was determined by measuring the lengths of {210}r intergrowths in a 10 µm × 10 µm scanned area at eight different positions across the sample surface, averaging the measured lengths, and normalizing the data to that of the images with highest line density. Thus, the maximum line density is shown to be unity. The error bars in Figure 5 represent the standard deviation of the eight measurements made for each condition. One possible explanation for the trend observed for sample B (Figure 5) is that spin-coating may initially produce an inhomogeneous distribution of Ga2O3 on the rutile surface. If this is the case, surface diffusion, which is generally faster than bulk diffusion, will dominate initially and result in an increase in the density of BGR intergrowth lines across the surface. As Ga2O3 becomes uniformly distributed across the surface, the driving force for surface diffusion will diminish, bulk diffusion will begin to dominate, and the density of intergrowth lines will decrease as Ga2O3 is transported into the bulk of the TiO2 crystal. (The observed defects lines are the edges of defect planes that extend beneath the surface.) Figure 5 also includes data from sample A collected after it was heated at 1350 °C. In this graph, the first data point for sample A is shown at 323 h, corresponding to 227 h at temperatures between 900 and 1300 °C plus 96 h at 1350 °C. Considering the difference in initial heating conditions of the two samples, one would not necessarily expect the data for sample A to continue the trend observed for sample B as is suggested
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Figure 2. AFM height images (10 × 10 µm) of (a) bare single-crystal [001] TiO2 (z range 100 nm), (b) BGR fired at 900 °C for 24 h (z range 100 nm), (c) BGR fired at 1100 °C for 24 h (z range 100 nm), and (d) BGR fired at 1350 °C for 96 h (z range 100 nm).
Figure 3. (a) SEM micrograph of BGR surface and (b) Auger electron spectroscopy overlay of (a), which identifies the presence of Ga (bright areas) residing along {210}r boundaries along [001] TiO2 surfaces.
by Figure 5. Nevertheless, the apparent agreement between the two data sets may reflect the role of surface diffusion in the early stages of the process. Typically, surface diffusion has a relatively low activation energy compared to bulk diffusion; thus initially heating at different temperatures may not have significantly affected the rate at which Ga2O3 was distributed across the surface. Perhaps the most important observation regarding the data of sample A is that the number of intergrowth lines appears to approach a constant level with increased heating, as is to be expected for diffusion from a finite thin-film source. The majority of BGR substrates used for DNA attachment studies were fired at 900 °C for 3 h and then at 1350 °C for 96-480 h. This resulted in substrates with a root-mean-square roughness of ∼1.5 nm, as determined by the average of several 2 × 2 µm AFM scans. This is approximately 5-6 times the roughness measured for bare mica substrates using the same technique.
DNA Attachment Studies. A number of different application methods were explored as a means of depositing DNA on BGR and [001] rutile substrates. The results of these trials are summarized in Tables 1 and 2. BGR and rutile substrates were exposed to 20 µL of DNA buffer solutions for 10 min and dried by wicking with filter paper and with compressed air. With the exception of BGR substrates exposed to Ni(II) solutions (Figure 6), attachment of DNA was too weak to allow AFM imaging. A second set of wicked samples was prepared in which substrates were pretreated for 5 min with the various salt solutions. Again, attachment was only observed on BGR substrates in the presence of Ni(II) cations. In both cases, DNA attached randomly on the BGR surface, showing no preference for the intergrowth regions. DNA did not attach to the rutile substrates. For most of the solutions examined, the force of attachment between the substrate and the DNA molecule was too small to overcome the forces of wicking.
AFM Study of DNA-BGR Interactions
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Figure 4. Preferential alignment of intergrowth boundaries observed in (a) AFM height image (10 × 10 µm) of BGR fired at 1350 °C for 96 h (z range 100 nm) and (b) AFM height image (10 × 10 µm) BGR fired at 1350 °C for 132 h (z range 100 nm). Table 2. Summary of DNA Attachment to TiO2 Surfaces DNA attached amt of solution applied surface cation bare bare TiO2 pretreated pretreated (µL) exposure addition TiO2 (rinsed) TiO2 (rinsed) TiO2
Figure 5. Relative line density as a function of heating time. Sample B was heated at 1350 °C. Sample A was initially heated at 9001300 °C (data not shown) and then heated at 1350 °C (data shown). Table 1. Summary of DNA Attachment to BGR Surfaces amt of solution applied (µL)
surface exposure
cation addition
bare BGR
20 20 20 20 20 20 20 20 20 10 10 10 10 10 10 10 10 10
wicked wicked wicked wicked wicked wicked wicked wicked wicked dried dried dried dried dried dried dried dried dried
none Ca(II) Co(II) Cu(II) Fe(II) Mg(II) Mn(II) Ni(II) Zn(II) none Ca(II) Co(II) Cu(II) Fe(II) Mg(II) Mn(II) Ni(II) Zn(II)
no no no no no no no yes no no no yes no no yes yes yes yes
DNA attached? bare BGR pretreated (rinsed) BGR
trace
trace trace trace yes trace
no no no no no no no yes no no no yes no no yes yes yes yes
20 20 20 20 20 20 20 20 20 10 10 10 10 10 10 10 10 10
wicked wicked wicked wicked wicked wicked wicked wicked wicked dried dried dried dried dried dried dried dried dried
none Ca(II) Co(II) Cu(II) Fe(II) Mg(II) Mn(II) Ni(II) Zn(II) none Ca(II) Co(II) Cu(II) Fe(II) Mg(II) Mn(II) Ni(II) Zn(II)
no no no no no no no no no no no no no no no no yes no
trace
no no no no no no no no no no no no no no no no yes no
trace
pretreated BGR (rinsed)
trace
trace trace trace yes trace
Additional experiments were prepared with solutions deposited for periods of 20 and 30 min prior to wicking, but similar results were obtained. To allow a greater amount of time for DNA interaction with the surface and to avoid the forces associated with wicking, 20 µL aliquots of the DNA buffer solutions were allowed to dry completely on the BGR substrates. The dried 20 µL aliquot masked most of the BGR surface features and prevented imaging of DNA attachment, if any. Figure 7a shows the edge of the dried
Figure 6. AFM height image (1 × 1 µm, z range 10 nm) of DNA molecules bound to a BGR surface. The DNA/NiCl2 buffer was exposed to the BGR surface for 10 min and then wicked with filter paper and allowed to dry.
salt coating on the BGR surface. The samples were then gently rinsed with distilled water, allowed to dry, and reimaged. Little to no DNA was observed on these sample surfaces. Occasionally, DNA molecules were observed on the sample surface bound to
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Figure 7. (a) AFM height image (50 × 50 µm, z range 300 nm) of the edge of a buffer coating on a BGR surface. (b) AFM height image (900 × 900 nm, z range 6 nm) of a washed BGR surface where DNA molecules have been pulled in the direction of a receding meniscus.
Figure 9. AFM image (1 × 1 µm) of DNA attached to a [001]oriented single-crystal TiO2 surface in the presence of Ni(II) (height image, z range 15 nm).
Figure 8. AFM images (1 × 1 µm) of DNA attached to BGR surfaces in the presence of (a) Co(II) (phase image, z range 35°), (b) Mg(II) (phase image, z range 40°), (c) Mn(II) (height image, z range 10 nm), (d) Ni(II) (phase image, z range 30°), and (e) Zn(II) (phase image, z range 80°) cations.
surface irregularities, believed to be undissolved salt (Figure 7b). Portions not in direct contact with irregularities appear to be pulled in the direction of the receding meniscus, as shown in Figure 7b. The amount of DNA buffer solution applied to the substrates was reduced to 10 µL. Unlike the 20 µL trials, dried 10 µL aliquots allowed imaging of the underlying substrate. As shown in Figure 8, DNA was bound strongly enough to the BGR surfaces in the presence of certain divalent cation salts (CoCl2, MgCl2,
MnCl2, NiCl2, and ZnCl2) to allow the imaging of the attached molecules. DNA molecules were not observed in systems when CaCl2, CuCl2, and FeCl2 additions were incorporated. For TiO2 substrates (both bare and pretreated), attachment of DNA to TiO2 was only observed when buffers contained Ni(II) ions (Figure 9). When buffers with no salt additions (blanks) were used, AFM images showed no evidence of attachment on bare or even saltpretreated surfaces. In the absence of a divalent cation, the attachment force between DNA and the BGR surface was insufficient to withstand wicking, rinsing, and tip forces. The null result validates the importance of the salt additions in contributing to the attachment of DNA to the BGR surface. To quantify whether DNA was preferentially attaching to {210}r intergrowth lines in the presence of divalent cations, the density of DNA molecules attached to {210}r intergrowth lines was compared to that of the density of DNA molecules crossing the equivalent length of lines drawn randomly across the AFM image. In this context, the density of DNA molecules is defined as the number of molecules per micrometer. The random lines were selected by placing an orthogonal grid with sequentially numbered grid lines, 1.25 mm apart, atop the AFM image and using a random number generator to pick an appropriate number of grid lines, along which DNA counting would be conducted.
AFM Study of DNA-BGR Interactions
Figure 10. Comparison of the density of DNA attached to {210}r intergrowths (solid bars) to the density of DNA observed along random lines along the surface of BGR substrates (hatched bars) for different divalent cations. Data suggest a preference of DNA to attach along {210}r regions.
For each image, the random-line counting was repeated eight times and averaged. Intergrowth-line and random-line counts were made for all images collected and averaged to yield the results for the different cations examined. The number of images collected typically ranged from 20 to 30, except for those collected for Mn(II)-treated samples system where only three images were available for counting. Figure 10 compares the average density of DNA counted along the {210}r lines to the average density counted along random lines in the AFM images. Significant variations are noted in the counted data, which can be attributed in part to problems associated with properly discerning long, single molecules of DNA from linked and/or aggregated smaller molecules. Moreover, the roughness of the substrate hindered the identification of DNA molecules in images with large scan sizes. Despite the large variations, we conclude that DNA attaches preferentially, although not exclusively, along the intergrowth lines since the density along intergrowth lines was consistently higher than that along the random lines for all of the cations that facilitated DNA binding to BGR surfaces, i.e., Co(II), Mg(II), Mn(II), Ni(II), and Zn(II). The two smallest two cations tested (Ni(II) and Mg(II)) yielded samples with the highest DNA densities, but there was no apparent relationship between the degree of preferential attachment and the size of the cations. Of all the cations examined in this study, Ni(II) was the most effective in binding DNA to the substrate surfaces. Among the wicking trials on BGR substrates, only those prepared with Ni(II) salts showed evidence of DNA attachment. For dried samples, only those prepared with Ni(II) displayed a significant amount of DNA attachment after rinsing. Moreover, Ni(II) was the only cation tested that promoted significant DNA attachment to [001] TiO2 substrates. The Ni(II) cation was also found to be the most effective in binding DNA to mica surfaces, which was attributed to the cation’s size and relatively high hydration enthalpy.14 As mentioned previously, DNA attachment was not observed on BGR substrates when Ca(II), Cu(II), and Fe(II) were used as the divalent cation. Calcium(II) was the largest cation investigated in this study, with an ionic radius of 0.99 Å compared to a range of 0.69-0.83 Å for the other cations.30 In studies of DNA attachment on mica, Hansma et al. reported similar results and suggested that Ca(II) was too large to occupy mica cavities.14 Cu(II) and Fe(II) ions possess ionic radii within the range of cation radii observed to promote DNA binding to the BGR surface, suggesting that factors other than cation size influence attachment. Hansma et al. reported previously that buffers containing Cu(II) (30) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
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additions led to lumps of DNA instead of strands on mica surfaces.14 Other studies have shown that Cu(II) and Fe(II) ions can cause chain mutation, cleavage, and denaturation,31-35 all of which could negatively influence the molecules’ ability to bind to a solid surface. The mechanism(s) responsible for binding DNA to the BGR surface is (are) not yet well understood. As mentioned previously, tunnel sites along the intergrowth lines are suitable hosts for small to medium-sized cations. The preferential attachment of DNA at the observed intergrowth lines provides some evidence to support the notion that a divalent cation residing at the tunnel site may facilitate binding, e.g., via an electrostatic bridge as has been proposed previously.14-16 It is interesting to note that, except for the deposition solutions containing Ni(II), DNA did not attach strongly enough to rutile substrates to allow imaging but was observed on the rutile portion of the BGR substrate. It is possible that isolated tunnel sites or very short intergrowth lines cannot be detected by AFM imaging and that the apparent binding on the rutile portion of the BGR actually involves a tunnel site, but more evidence is required to support this notion.
Conclusions BGR intergrowth substrates suitable for DNA attachment studies can be prepared by spin-coating gallium isopropoxide onto [001]-oriented TiO2 single-crystal slabs and heating above 1350 °C. This generally results in the random orientation of intergrowth lines among different members of the {210}r family of planes. In some instances, the parallel alignment of the intergrowth lines was observed, but the factors that result in such arrangements are not well understood. Further development of the process will be required to achieve controlled intergrowth arrangements, i.e., patterned substrates. Additionally, methods to achieve a flatter, smoother surface are desired. The deposition of DNA on [001] rutile and BGR surfaces from buffer solutions containing various divalent cations was studied. Of all the cations used, only Ni(II) facilitated the attachment of DNA on rutile, albeit in very small quantities. DNA attachment to BGR surfaces was strong enough to allow AFM imaging when the deposition buffer contained one of the following cations: Co(II), Mg(II), Mn(II), Ni(II), and Zn(II). For all of these cations, DNA attachment occurred preferentially, but not exclusively, along BGR intergrowth lines. When buffers without cation additions and those containing Ca(II), Cu(II), and Fe(II) were used, DNA failed to bind the BGR surfaces strongly enough to allow AFM imaging. The mechanism(s) by which DNA attaches to the BGR surface is (are) not well understood, but may involve the incorporation of divalent cations at the tunnel sites of the BGR intergrowths. Acknowledgment. This work was supported by the National Science Foundation (DMR-0093690). N.E. was funded by the National Defense Science and Engineering Graduate (NDSEG) Fellowship program. The work was conducted using the facilities of Alfred University’s Advanced Research Center in Photonics Education and Training, made possible by a grant (C00070) from the New York State Office of Science, Technology, and Academic Research (NYSTAR). LA060206Z (31) Tkeshelashvili, L.; et al. J. Biol. Chem. 1991, 266, 6401. (32) Saenger, W. Principles of Nucleic Acid Structure; Springer-Verlag: New York, 1984. (33) Eichhorn, G.; Clark, P. Proc. Natl. Acad. Sci. 1965, 53, 586. (34) Eichhorn, G. L.; Shin, Y. A. J. Am. Chem. Soc. 1968, 90, 7323. (35) Rai, P.; Wemmer, D.; Linn, S. Nucleic Acids Res. 2005, 33, 497.
